Partnership and S Corporation Filing Deadlines and Extensions: What the Code Actually Requires

Most business owners think of April as tax season. If you own a partnership, a multi-member LLC, or an S corporation, that assumption is expensive. Your entity's return is due a full month earlier, and the penalty for missing it is charged per owner, per month, whether or not the business owes a dime of tax.

This guide walks through the actual due dates, how the automatic extension works, and what happens when the date slips by.

The Statutory Due Date: The 15th Day of the Third Month

The deadline comes from IRC Section 6072(b). Partnership returns required by IRC 6031 and S corporation returns required by IRC 6037 that are filed on a calendar-year basis are due on or before March 15 following the close of the year. Fiscal-year entities file by the 15th day of the third month after the fiscal year ends.

When the 15th lands on a weekend or legal holiday, the IRS instructions treat a return filed the next business day as timely. That is why the 2025 Form 1065 and Form 1120-S instructions list March 16, 2026 as the deadline for calendar-year 2025 returns: March 15, 2026 fell on a Sunday.

Return	Code section requiring it	Calendar-year due date	Automatic extension
Form 1065 (partnerships and multi-member LLCs taxed as partnerships)	IRC 6031	March 15	6 months, Form 7004
Form 1120-S (S corporations)	IRC 6037	March 15	6 months, Form 7004

One detail people miss: the IRS does not care whether the entity made money. A partnership that lost money still has to file. The return exists to tell the IRS what each owner should be reporting.

Schedule K-1 Timing Is Tied to the Return

The entity return is only half the job. Each owner also needs a Schedule K-1.

For partnerships, IRC 6031(b) and Treas. Reg. 1.6031(b)-1T(b) require the partnership to furnish each partner's statement on or before the day the partnership return is required to be filed, determined with regard to extensions. So if the partnership properly extends, the K-1 deadline moves with it.

For S corporations, IRC 6037(b) requires the corporation to furnish each shareholder a copy of the required information on or before the day the return was filed. Either way, the K-1s and the return travel together. Missing the K-1 deadline triggers its own penalty under IRC 6722, which we cover in our guide to failure to furnish K-1 penalties.

How the Form 7004 Extension Works

Both entity types get an automatic six-month extension by filing Form 7004. The rules are in the regulations, and they are short:

• Partnerships: Treas. Reg. 1.6081-2 grants an automatic six-month extension if the partnership files a complete Form 7004 on or before the return's due date. No additional extension is allowed beyond those six months.
• S corporations: Treas. Reg. 1.6081-3 grants an automatic six-month extension to corporations that file Form 7004 by the due date. That regulation also requires a corporation to remit its properly estimated unpaid tax by the payment due date, and it states plainly that the extension does not extend the time to pay.

"Automatic" means you do not need the IRS to approve it. It does not mean you can file it late. A Form 7004 filed after the original due date does nothing for you, and the penalty clock is measured from the original deadline.

There is one more wrinkle. Under both regulations, the IRS can terminate an automatic extension by mailing a notice at least 10 days before the termination date. That is rare, but it means the extension is a privilege, not a vested right.

The Entity Extension Does Not Extend Your Personal Return

Treas. Reg. 1.6081-2(e) says it directly: an extension for the partnership return does not extend the time for filing a partner's own income tax return or paying the tax due on it. Owners waiting on a late K-1 need to deal with their own deadline separately. If that describes you, deal with your own return on its own timeline rather than waiting on the business.

What Missing the Deadline Costs

A late Form 1065 is penalized under IRC 6698. A late Form 1120-S is penalized under IRC 6699. The structure of the two penalties is the same:

• A base dollar amount per month (or part of a month) the return is late,
• multiplied by the number of persons who were partners or shareholders at any time during the year,
• for up to 12 months.

The statute sets the base at $195 and indexes it for inflation. Rev. Proc. 2024-40 sets the base at $255 for returns required to be filed in 2026 (that is, 2025 calendar-year returns), and the 2025 Form 1065 and Form 1120-S instructions state the same $255 figure. Rev. Proc. 2025-32 sets the base at $260 for returns required to be filed in 2027.

Run the numbers on a modest business. A four-member LLC that files its 2025 Form 1065 a full year late faces $255 times 4 partners times 12 months, or $12,240. No tax was owed. The penalty is purely for not filing.

The same penalty applies if the return is filed on time but leaves out required information. IRC 6698(a)(2) and IRC 6699(a)(2) treat an incomplete return the same way as a late one. For S corporations that owe entity-level tax, the Form 1120-S instructions note that the regular late-filing addition based on unpaid tax can apply on top of the per-shareholder penalty.

Two more points matter here. First, the penalty is assessed against the entity itself (IRC 6698(c), IRC 6699(c)). Second, deficiency procedures do not apply to these penalties (IRC 6698(d), IRC 6699(d)), so the IRS can assess them directly once the return posts late. For a detailed breakdown, see our pages on the Form 1065 late filing penalty and the Form 1120-S late filing penalty.

If You Already Missed It

Do not compound the problem by waiting. The per-month penalty keeps running until the return is filed or the 12-month cap is reached. Get the return in first.

Then look at relief. The Internal Revenue Manual lists the late-filing penalties under IRC 6698(a)(1) and IRC 6699(a)(1) among those eligible for First Time Abate (IRM 20.1.1.3.3.2.1), provided the entity has a clean filing history for the prior three years. Reasonable cause is another route. Small partnerships may qualify for relief under Rev. Proc. 84-35, which the IRS reproduces in Exhibit 20.1.2-1 of the IRM. The 2025 Form 1065 instructions also tell partnerships not to attach an explanation to the late return; send it after the penalty notice arrives.

Start with our guide to First Time Abatement for entity returns. If you are several years behind, the playbook is different; see catching up unfiled 1065 and 1120-S returns.

A Simple Calendar That Prevents Most Problems

1. January: Confirm the books are closed and every owner's address and ownership percentage are current.
2. By mid-February: Decide whether the return will be ready. If there is any doubt, plan to extend.
3. By the 15th day of the third month: File the return and furnish K-1s, or file Form 7004.
4. By the extended due date: File the return and furnish K-1s. There is no second extension.

A single missed March deadline can snowball into notices, owner-level problems, and years of catch-up work. You can read more about how those problems spread in this overview of IRS problems for small businesses.

If you are already facing penalty notices or unfiled entity returns, call the Law Offices of Darrin T. Mish, P.A. in Tampa, Florida at 813-229-7100. The calendar does not care how busy you were in March. The IRS doesn't either.